io: change AsyncRead to use a ReadBuf (#2758)

Works towards #2716. Changes the argument to `AsyncRead::poll_read` to
take a `ReadBuf` struct that safely manages writes to uninitialized memory.

tokio-util/src/compat.rs

@@ -1,5 +1,6 @@
 //! Compatibility between the `tokio::io` and `futures-io` versions of the
 //! `AsyncRead` and `AsyncWrite` traits.
+use futures_core::ready;
 use pin_project_lite::pin_project;
 use std::io;
 use std::pin::Pin;
@@ -107,9 +108,18 @@ where
     fn poll_read(
         self: Pin<&mut Self>,
         cx: &mut Context<'_>,
-        buf: &mut [u8],
-    ) -> Poll<io::Result<usize>> {
-        futures_io::AsyncRead::poll_read(self.project().inner, cx, buf)
+        buf: &mut tokio::io::ReadBuf<'_>,
+    ) -> Poll<io::Result<()>> {
+        // We can't trust the inner type to not peak at the bytes,
+        // so we must defensively initialize the buffer.
+        let slice = buf.initialize_unfilled();
+        let n = ready!(futures_io::AsyncRead::poll_read(
+            self.project().inner,
+            cx,
+            slice
+        ))?;
+        buf.add_filled(n);
+        Poll::Ready(Ok(()))
     }
 }
 
@@ -120,9 +130,15 @@ where
     fn poll_read(
         self: Pin<&mut Self>,
         cx: &mut Context<'_>,
-        buf: &mut [u8],
+        slice: &mut [u8],
     ) -> Poll<io::Result<usize>> {
-        tokio::io::AsyncRead::poll_read(self.project().inner, cx, buf)
+        let mut buf = tokio::io::ReadBuf::new(slice);
+        ready!(tokio::io::AsyncRead::poll_read(
+            self.project().inner,
+            cx,
+            &mut buf
+        ))?;
+        Poll::Ready(Ok(buf.filled().len()))
     }
 }